STEVEN Avery, the man at the centre of the world’s most talked-about true crime series, has broken his long silence from behind bars, warning: “the real killer is still out there”.

Avery, whose incredible run-ins with the law are the focus of the Netflix docuseries Making a Murderer, is currently serving life without parole for the brutal 2005 murder of Wisconsin photographer Teresa Halbach.

In a three-page letter to Wisconsin news reporter Colleen Henry, 53-year-old Avery maintained his innocence of a crime that horrified Manitowoc County and also landed his nephew, Brendan Dassey, in jail.

“The real killer is still out there,” Avery wrote.

“Who is he stalking now? I am really innocent of this case and that is the truth!!! The truth will set me free!!!!!!!”

Avery has been convicted for the 2005 murder of photographer Teresa Halbach, 25, but he maintains his innocence. Picture: Halbach family via Herald TimesSource:AP

Avery was arrested and charged with Halbach’s murder just two years after he was exonerated over a 1985 sexual assault for which he spent 18 years in jail. In his letter, Avery urged Henry to investigate the sheriff of Manitowoc County, one of several law enforcement figures he believes framed him for the murder in order to get back at him for his $US36 million ($A51 million) wrongful imprisonment lawsuit.

She stood by him for most of his trial but Steven Avery's ex Jodi Stachowski now says he’s guilty. Picture: HLNSource:Supplied

According to Milwaukee news agency WISN 12, Avery agreed to an interview with Henry but the Department of Corrections said he was not allowed to.

Avery and his new legal team are still working on proving his innocence.

According to WISN 12, he filed an appeal last week claiming his property was illegally searched and asked for a new trial and to be released.

The state responded by saying Avery was a flight risk and shouldn’t be released from custody.

Making a Murderer, a 10-part series that casts doubt on Avery’s trial and the sheriff department’s role in handling Halbach’s murder investigation, has left millions of viewers debating whether or not Avery truly belonged behind bars.